U.S. Pat. No. 4,453,919 discloses an air-driven vibrator having a body defining a disc-shaped working chamber in which a disc-shaped vibratory element or rotor made of steel is received for oscillatory movement, a plurality of air injection ports being arranged tangentially to the working chamber to inject compressed air therein to thereby excite the vibratory element, the vibrator further having an exhaust port for discharging used air. The vibrator may be incorporated, for example, in a dental scaler to oscillate a vibratory tool adapted to remove calculus or plaque from teeth.
Although the mechanism of vibration generation taking place in this vibrator is not clear, upon injection of compressed air tangentially into the working chamber to produce a swirling air stream therein, the vibratory element rotates or oscillates like a coin wobblingly spinning on a table surface and strikes the end walls of the working chamber thereby generating vibration.
When after use the supply of compressed air is terminated, the vibratory element, no longer excited, will cease to rotate and will therefore fall vertically downwards within the working chamber under the action of the gravity. The particular part of the wall of the working chamber upon which the vibratory element will finally rest and the angular position in which the vibratory element will finally repose will be dictated by the angular position of the working chamber relative to the vertical.
If, when the vibrator is not operated, the vibrator is held in such a position that the inner end wall of the working chamber lies in a generally horizontal plane so that the vibratory element similarly lying in a substantially horizontal position rests upon the inner end wall of the working chamber, as shown in FIG. 6A of U.S. Pat. No. 4,453,919, there is a risk of the vibratory element to inadvertently adhere to or "stick" to the inner wall of the working chamber as a result of one of the axial end faces of the element being in snug contact with the inner end wall of the chamber.
The risk of the vibratory element to stick will be increased particularly when the vibrator is properly lubricated because a film of lubricant formed and existing between the end face of the element and the inner end wall of the chamber promotes adhesion. Formation of moisture condensate on the inner walls of the working chamber similarly tends to assist sticking.
Furthermore, it is desirable that the end faces of the vibratory element and the inner end walls of the working chamber be finished by a lapping machine in order to enhance the degree of dimensional precision thereof. However, the specular surfaces of the element and chamber walls thus formed by lapping finishing would cause the vibratory element to stick more strongly to the chamber walls.
In the event that the vibratory element once sticks to the inner walls of the working chamber, the vibratory element would not resume its movement merely by reopening the air supply. To restart the vibrator, the vibratory element must be separated or detached away from the inner walls of the working chamber by hammering the vibrator or otherwise giving impact thereon.
U.S. Pat. No. 4,453,919 solved this problem of sticking by conically tapering the axial end faces of the vibratory element at a small angle by chamfering along the circumferential edges of the element, as shown in FIGS. 5A-5H and FIG. 6B thereof, to ensure that the vibratory element is separated from the inner walls of the working chamber by air jets blown into the wedge-shaped space formed between the conical end face of the element and the flat inner wall of the working chamber.
Hitherto, the chamfering of the disc-shaped vibratory elements has been carried out by using a cylindrical grinding machine.
More specifically, a disc-shaped unfinished vibratory element 1 prepared by cutting a steel rod is chucked by a chucking device 2 of a rotary workpiece holder of the cylindrical grinding machine as shown in FIG. 1 of the accompanying drawings, and the axis 3 of the workpiece holder is positioned at an angle, for example, of 88.degree. relative to an axis 5 of a grinder wheel 4 as shown in FIG. 2. Then the vibratory element 1 as chucked by the chuck 2 is translated forward toward the rotating grinder wheel 4 as the element is rotated about the axis 3 of the workpiece holder. In this manner, one of the end faces of the element 1 is ground and chamfered conically at a small angle of 2.degree..
Then, the semifinished vibratory element is detached from the chuck of the workpiece holder and, after being turned over for 180.degree., the element is again chucked and ground to chamfer the other end face.
However, the problem encountered in the conventional process for chamfering the vibratory elements is that the vibrators used and incorporated in dental handpieces are so small in size that the axial thickness of the vibratory elements is extremely thin, it being on the order of only 1 mm. Accordingly, it has been difficult to properly chuck the vibratory elements in the workpiece holder of the cylindrical grinding machine. In particular, it has been extremely difficult to chuck those elements having one of the end faces already subjected to chamfering.
Because of the difficulty in properly chucking the vibratory elements, it has often been encountered that the axis of the element as chucked in the chuck is misaligned with the axis of the workpiece holder of the grinding machine. This has precluded to perform chamfering accurately. As a result, a large number of defective elements have been produced so that the production yield of the vibratory elements has been extremely low.
Furthermore, it has been necessary to carry out chamfering of the vibratory elements manually one by one. This has resulted in a fluctuation in the quality of the finished elements, thereby also giving rise to low production yield. In addition, chamfering by manual operation is costly and time consuming and precludes manufacturing by the mass production process.